The present invention relates to a tension leg platform for petroleum recovery on the sea bed, consisting of a platform deck supported by a buoyancy structure, a bottom anchoring structure and tension elements extending between the buoyancy structure and the bottom anchoring structure.
The buoyancy of such a platform exceeds its weight by a margin termed the excess buoyancy, which maintains the tension elements in tension. Tension leg platforms have been described in U.S. Pat. No. 3,648,638, GB patent specification No. 1.337.601 and GB Patent Specification No. 1.462.401.
The tension suppresses movement in a vertical plane, that is roll, pitch and heave, but only to a limited extent acts as a restoring force to reduce movement in a horizontal plane.
Where a platform is used for oil production from an underwater well, it is desirable to be able further to reduce the horizontal movement of the platform in relation to the well, for instance to facilitate the making of pipe, particularly riser, connections and to prevent damage thereto.
Generally, it is an object to provide the least possible movement in a seaway, and particularly to reduce the horizontal movement of the platform.
According to GB patent specification No. 1.462.401 this is achieved by means of thrusters. FIG. 1 of this patent specification illustrates a traditional tension leg platform in which the main buoyancy is provided by shafts which are only interconnected by means of bracings to provide sufficient strength. In order to illustrate that thrusters can be used irrespective of the shape of the buoyancy structure FIGS. 2 and 3 of the GB patent specification No. 1.462.401 show further suggested designs of the buoyancy structure. However, these designs are believed to be purely theoretical desk designs which will not function in practice.
It has also been suggested to position the shafts providing the main buoyancy rather close together and to connect them by means of upright cylindrical cells which fill the space between the shafts. The required stability is obtained by extending the shafts further down into the water in order to form a ballast structure.